High Nitrates a Concern in Stressed Forage Crops

By, Jordan Johnson, M.Sc., Agri-Environmental Specialist, Swift Current
Maddy Lazurko, AAg, Livestock and Feed Extension Specialist, Swift Current

With much of the growing season on the prairies occurring under extremely dry conditions, poor forage yields have urged many producers to consider the use of annual crops as forage sources for their cattle herds. When harvested as greenfeed, cereal crops such as oats, barley, wheat, rye or triticale can be comparable in feed quality to hay. While annual cereal crops are an attractive alternative to traditional forage sources, stressed growing conditions due to lack of moisture this season have resulted in many crops accumulating higher than normal levels of nitrates.

Nitrate accumulation in plants occurs whenever normal growth is disrupted. Under normal growing conditions, nitrates absorbed from the soil are converted to ammonia and incorporated into plant proteins. Stress caused by limited moisture, hot winds, hail, frost, spray drift or cool cloudy weather results in restriction of plant growth, but nitrate continues to be absorbed from the soil. This continued absorption from the soil results in accumulation of nitrates in plant, with concentrations being highest in the lower third of the stem.

Annual crops such as oats, barley, wheat, rye or triticale are most susceptible to nitrate accumulation. Some weeds such as kochia, thistle, millet, lambs quarters and pigweed are also known to be accumulators of high levels of nitrates. In addition to adverse weather and plant species, fields with high levels of soil nitrogen from nitrogen fertilizer or manure application may predispose plants to nitrate accumulation. Nitrate concentrations are typically highest in young plants and tend to decrease as plants mature, but plants grown under persistently stressful conditions or in soils exceedingly high in nitrogen may maintain high nitrate levels even at maturity.

Unlike annual crops, perennial forages or native grasses are less likely to accumulate nitrates since they are fertilized less frequently, ultimately resulting in lower soil nitrogen available for absorption. Thanks to their nitrogen fixing capabilities, legumes such as alfalfa, peas, lentils or faba beans are also unlikely to accumulate problematic levels of nitrates.

Feed testing is a critical component of establishing a feeding program for forages suspected of being high in nitrates. Under normal conditions, rumen bacteria convert feed nitrates to nitrite in the rumen, which is then converted to ammonia and utilized by microbes to create protein in the rumen. Nitrate toxicity occurs when rumen bacteria convert nitrates to nitrite more rapidly than nitrite can be converted to ammonia, resulting in an accumulation of nitrite in the rumen. When absorbed into the bloodstream, nitrite binds to hemoglobin and reduces the oxygen carrying capacity of blood. Total feed nitrate levels below 0.5 per cent are considered safe to feed, with feeds containing 0.5 to 1 per cent nitrate requiring careful adaptation and feeding management.

For more information on nitrates, visit this fact sheet on nitrate toxicity. To discuss concerns over high nitrates or to have your feed tested, please contact your local Ministry of Agriculture regional office or call the Agriculture Knowledge Centre at 1-866-457-2377.

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